This invention relates generally to a high speed multiple section computed-tomographic (CT) medical scanning system, and more particularly the invention relates to an electron beam target structure for use therein.
Disclosed in U.S. Pat. No. 4,352,021 is a high speed X-ray scanning system in which the X-ray source and the X-ray detectors are stationary and a plurality of fan beams of radiation is generated by sweeping an electron beam across a plurality of targets arcuately arranged whereby each target generates radiation fan beams.
The electronic scanning system incorporates a single electron beam tube. The electron beam is deflected by suitable mangetic and/or electric fields to produce a movable X-ray source on one of four adjacent semi-circular target rings to provide scanning fan beams that can be used to image an entire volume of tissue in multiple sections. Such an electronic scanning system is vastly superior in speed to the mechanical scanning systems in the prior art refences in U.S. Pat. No. 4,352,021. Fraction-of-a-second scan time of a volume can be achieved as compared to one or more seconds required for the mechanical scan of a single section. The system eliminates the need for moving parts that require high precision and alignment. In addition, elaborate systems of sliding electrical contacts are eliminated. The scanner is an improvement over that shown and described in U.S. Pat. No. 4,158,142, in that it permits nearly simultaneous viewing multiple sections of the body which may encompass a region as large as the heart. The scanner can provide as many as eight sections.
The system employs a plurality of detectors mounted opposite the target rings. The detectors are arranged in two adjacent partial-circular ring arrays. Each of the arrays contains a multiplicity of detectors as, for example, 432 detectors each, providing a total of 864 detectors. The angular separation of two adjacent detectors is in the order of 0.5 degrees resulting in very high resolution. The scanning system is provided with collimators both for the X-ray source and for the detectors. The source collimator provides a fan-shaped beam 30.degree. opening angle. The detector collimators provide interchangeable options: dual section detector arrays, single section detector arrays and high resolution single section detector arrays. A variety of scanning modes can be selected with up to eight sections being scanned at a rate of at least one scan per second.
A problem encountered in the described system results from the high temperatures of the electron beam target which can cause burnout or degraded X-ray sources. Typically; the electron beam current is one ampere at 120 Kv, and the beam power of 120 kilowatt, concentrated in a focus of a typical area of one mm by 10 mm, heats the target surface temperature to an extremely high value, even when this spot is moved along the targets with high speed.
U.S. Pat. No. 4,531,226 discloses a target structure including an arcuate frame and a plurality of support brackets extending therefrom. Each support bracket has at least one recessed portion including one inclined surface at an angle required by the surface of an electron beam target. Since the bracket is arcuate the inclined surface is in fact of conical shape. The electron beam target comprises a member having a planar surface. The member is supported in the recessed portion with the planar surface engaging the inclined recessed wall. Fastening means engages the target and maintains the planar surface of the target in forced engagement with the inclined recessed surface. The shape of the target takes on the conical shape of the inclined bracket surface. The fastening means accommodates targets of various thicknesses and allows for thermal expansion of the targets. Further, each target can be readily replaced in the field. Advantageously, in a multiple target arrangement each target shields the support bracket from the electron beam thereby reducing heat and minimizing damage to the bracket.
The '226 patent describes each target segment as being a solid tungsten sheet or a mounting substrate such as OFHC copper on which a tungsten layer is deposited by plasma spray or to which a tungsten sheet is brazed. Other X-ray emitting surface material such as tantalum and molybdenum may be used. However, a problem arises in attempting to braze the tungsten or molybdenum sheets to a supporting substrate and then forming the composite structure to a desired configuration.
Another object of the present invention is an improved composite electron beam target for use in an X-ray scanner.
Still another object of the invention is a method of making a composite electron beam target having a desired configuration.
A feature of the invention is preforming at least one individual metal layer and joining the layers by brasing in a die clamp.
Briefly, a composite structure is fabricated in accordance with the invention by preforming a substrate in a suitable mold. The preformed substrate and a layer of target material are then brazed in a die having a second preformed shape.
In accordance with a feature of the invention the dies have bi-radial cylindrical shapes to overcome the "flattening out" of the preformed substrate and the brazed composite sheets at the ends. Thus when the brazing die is opened the layers of brazed material will spring to the desired configuration and will maintain the configuration during usage.
In an electron beam target for producing a high brightness focus and emitting electrons, at least one of the layers is a refractory metal such as tungsten, tungsten alloy, molybdenum, or molybdenum alloy and the substrate material may be OFHC copper or a titanium-zirconium-molybdenum alloy. Because of high tungsten price, poor machining properties, brittleness, and high density those targets are often layered composites where only the active layer is tungsten (or tungsten alloy). The adjoined substrate material is selected in respect to the required properties for the joining technologies available and according to other desired properties like heat conductance, weight, machining properties, etc. (classical joining technologies are, among others, forging and brazing). The problems of brazing are numerous, dealing with differential expansion, wetting characteristics, as well as temperature stablility and cost of the brazing alloy, and more.
The invention and objects and feature thereof will be more readily apparent from the following detailed description and appended claims when taken with the drawing.